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    UPS Technical Glossary

    Here you will find the definitions of a number of technical terms used within the uninterruptible power supply (UPS) industry.


    AC Ripple

    Due to incomplete suppression of the alternating waveform after rectification, an AC ripple is harmful to a UPS battery causing internal heating and deterioration of the poles.

    Alternating current

    Typically used in power supplies, an alternating current (AC) reverses its electrical current direction numerous times a second at regular intervals in a sinewave.


    A measure of the electrical current flow (A)


    A measure of the number of Amps that a battery set can deliver

    Autonomy Rating

    Also known as back up or discharge time, battery autonomy is a measure of the time for which the battery will support the critical load during a mains failure. Autonomy is a function of battery charge state, capacity and load size.

    Automatic Transfer Switch (ATS)

    In the event of a significant loss of mains power, an automatic transfer switch will detect the power loss and transfer the load to the secondary supply line.

    Availability (A)

    Is a useful measure of downtime per year for systems subject to failure and repair. It is defined as the probability of a system being operational at any given time during its working life;

    A = MTBF/(MTBF + MTTR)

    Where MTBF = Mean Time Between Failures and MTTR = Mean Time To Repair



    A complete loss of electrical power

    Battery block

    A self contained battery consisting of a number of individual and connected battery cells.


    Battery cell

    A simple electrical circuit consisting of positive and negative electrodes or plates, electrolytes an a separator.


    A condition in which power supply remains at a low voltage for a long period of time.


    Acts as a power path around a UPS system allowing the safe removal of a UPS system for service, maintenance or replacement by switching the load without interrupting the power flow.


    Capacity System (N)

    A capacity or ‘N’ system is the most common type of UPS installation and the minimum requirement to provide power protection to the critical load. Also referred to as ‘power parallel’, It comprises a single standalone UPS module or a paralleled set of modules with a matched capacity to the critical load projection.

    Circuit Breaker

    A protective device that interrupts the flow of current when it exceeds a specified value.

    Coefficient of Capacity; Coefficient of Redundancy

    In an N+n parallel redundant system, N is the coefficient of capacity and n is the coefficient of redundancy. N represents the number of modules needed to meet the critical load and n is the number of extra, redundant modules.

    Current (A)

    Expressed as Amps, the current measured the volume of electricity flowing thorough a circuit.

    Critical loads

    Electrical systems which must be kept running at all times, even during a mains power failure, as they directly affect the companies ability to function.

    Crest factor

    The mathematical ratio between the crest (peak or maximum) value and the root-mean-square (RMS) value of an AC waveform. Some computer power supplies draw current with a crest factor of between two and three.

    Current Limit

    The restriction of the amount of current that can be drawn from any point within an electrical circuit or UPS output.


    Deep discharge

    The state of a battery whereby the voltage (Vdc) has dropped below a safe operating level from which the battery can not recover.

    Direct current (DC)

    An electric current which contains electrons going only in one direction.


    A transient voltage decrease – also called a sag.

    Double Conversion (online)

    In an online double conversion UPS system he internal rectifier converts incoming power from AC to DC, for the UPS battery, and then back to AC before reaching the load.

    Double Conversion Efficiency

    The double conversion efficiency figure for a UPS is obtained by comparing the output power to the load with the input mains power applied to the UPS, where both figures are in kW. This efficiency improves as the load approaches the maximum UPS capacity, so efficiency can be maximised by running the UPS as near to full load as possible.


    Environmental monitoring probe (EMP)

    A plug in device which works in tandem with an NMC card. The EMP allows users to monitor the temperature and humidity within the immediate vicinity of the UPS.

    External Battery Module (EBM)

    Extra battery strings mounted in modules allowing greater autonomy.


    Frequency converter

    A device for changing the input frequency to a different output frequency.

    Float charging

    A battery charging scheme suitable for UPS batteries, designed to maximise battery life.



    A generator automatically provides backup power within seconds of a mains power outage. An automatic transfer switch senses the power loss, commands the generator to start and then transfers the electrical load to the generator. Generators are usually fuelled by diesel, natural gas or liquid propane gas.

    Galvanic isolation

    Two circuits are ‘galvanically’ isolated if their only connection is a transformer coupling. A standard transformer based UPS design typically does not have galvanic isolation because the neutral line passes from the input via the static bypass switch to the output.



    Electrically separating a UPS systems input from output.

    Inrush current

    The initial surge in current drawn by loads, for example, the charge capacitive circuits.


    The circuit within the UPS that converts DC power back to AC


    kVA rating

    1kVA is 1000 volt amps. The key performance indicator of a UPS is the power factor. This helps to identify the UPS’ output potential in Watts or kW.



    the electrical device connected to a power source is referred to as the ‘load’. In terms of the UPS, the load is the required amount of current from the attached equipment.

    Line interactive UPS

    Mains power flows via the bypass line directly to the load. There may be a 5-10 millisecond gap before the UPS provides power if the mains fail.


    Maintenance bypass

    The maintenance bypass offers the flexibility to safely remove a UPS without disrupting power to the load, allowing for safe service, maintenance and replacement of UPS systems.

    Mobus card

    In a similar way to SNMP connectivity, the mobus card will allow the UPS to be monitored in real time, alerting the client of any power disturbances.


    A unit containing all hardware and software necessary for full system operation.



    Normally measured in decibels (dB) or (dBA). Noise may also be referred to as electrical noise in a circuit and refers to an undesirable electrical signal.


    Off-line UPS system

    During normal operating the load is power by mains electricity. If the mains power fails, the load is transferred to the UPS inverter output.

    On-line UPS system

    The batteries within an online UPS system are always connected to the inverter so there is no power interruption.


    An American term used to describe a mains power supply failure, also referred to as a blackout.


    Power factor

    The relationship between the stated ‘kVA’ rating and the amount of power required from the UPS. The power factor can be either lagging or leading, the higher the power factor the more output the UPS is capable of.



    The UPS is able to fit in a server rack saving valuable floor space in crowded server environments.


    Redundancy is a term often used to describe the state of being no longer needed or useful. However, when used in the context of UPS and critical power backup, redundancy refers to the duplication of critical components or systems, with the intention of increasing reliability as a whole to support business continuity.



    An SNMP card allows interface between the UPS unit and IT hardware enabling the protection of sensitive equipment during extended power outages. The SNMP card transmits data over the internal network monitoring for alarms and performance notifications based on a change in UPS status. The key information can be emailed or transferred via SMS.


    Short duration increases in voltage above the mains power supply nominal, which generally lasts for several cycles.


    Three phase

     A supply consisting of three single phase, each 120 degrees out of phase from one another.


     A wound component consisting of wingdings around a core, with an iron sheet laminated that can be used to change voltage levels and provide galvanic isolation.

    THDi (Total harmonic distortion)

    A measure of all the harmonics induced in a system compared to a normal sinewave.


    UPS (uninterruptible power supply)

    The terminology used to describe a system (either static or rotary) capable of maintaining power to a load for a defined time irrespective of the state of the mains power itself – also known as an uninterruptible power system.


    VRLA (Valve-regulated lead-acid) battery

    A type of lead-acid battery commonly used within a UPS which emits virtually no gas, required no topping up and needs no special ventilation.


    A measure of electrical force or pressure, which can be expressed as Vac or Vdc.